1. Field of the Invention
The present invention relates to a photoelectric conversion device, and more particularly, to a photoelectric conversion device used as an input unit for a facsimile machine, an image reader, a digital copying machine, an electronic blackboard or the like.
2. Related Background Art
Recently, long line sensors having optical systems with a one-to-one magnification ratio have been developed as photoelectric conversion devices for the purpose of providing small and high-performance facsimile machines, image readers, and the like.
Furthermore, for the purpose of providing small and inexpensive devices, photoelectric conversion devices have also been proposed in which a sensor directly detects light reflected from an original via a transparent spacer made of glass or the like without using a fiber lens array with a one-to-one magnification ratio.
FIGS. 1(A) and 1(B) schematically show an example of a photoelectric conversion device. The photoelectric conversion device has been proposed by a research group of the assignee in U.S. patent application Ser. No. 481,227 filed Feb. 20, 1990. FIG. 1(A) is a schematic cross-sectional view of the photoelectric conversion device, as seen from the main scanning direction of a photoelectric conversion element array. FIG. 1(B) is a schematic cross-sectional view of the device, as seen from the subscanning direction of the photoelectric conversion element array.
In FIGS. 1(A) and 1(B), a transparent sensor substrate 1 includes photoelectric conversion elements (not shown) formed on a transparent substrate, made of glass or the like, by a semiconductor process or the like, and an illuminating window (not shown). A transparent mounting substrate 2 includes an interconnection wiring member 4 formed on a transparent substrate, made of glass or the like, by a thick film printing method, a photolithographic method or the like. The interconnection member 4 electrically connects the transparent sensor substrate 1 to a driving circuit unit (not shown) provided on the transparent mounting substrate 2. The transparent sensor substrate 1 is bonded on the transparent mounting substrate 2 by an adhesive layer 5. A light source 3 for emitting light for illuminating an original P comprises an LED array having a plurality of LED chips 6 arranged in the form of an array.
The reading position rf the original P, the position of the illuminating window in the transparent sensor substrate 1, and the optical axis of the light source 3 in the direction of the array are set to positions which exist within a vertical plane descending from the reading position of the original P, as shown by arrow L.
Illuminating light L from the light source 3 is projected onto the original P after passing through the transparent mounting substrate 2 and the illuminating window within the transparent sensor substrate 1. Light reflected from the original P is incident upon the photoelectric conversion elements on the transparent sensor substrate 1, and is subjected to photoelectric conversion by the photoelectric conversion elements. The converted signal is output as an image signal.
However, in an attempt to provide a further inexpensive and smaller device, it has become clear that the above-described photoelectric conversion devices have the following technical problems.
As a means for providing an inexpensive photoelectric conversion device, it is possible to reduce the number of the LED chips and thus to reduce the cost of the light source.
When the number of the LED chips are reduced, the amount of light incident on the surface of the original is reduced, causing uneven illumination. This substantially deteriorates the quality of an image, especially when a white original is read.
In order to reduce the uneven illumination, it is possible to increase the distance between the LED array and the surface of the original. This approach, however, is by no means preferable, since the illumination substantially decreases, with distance and the photoelectric conversion device becomes large.
A plurality of LED chips can also be arrayed in a zigzag or a staggered form. As a result, there is a possibility that uneven light intensity will occur depending upon the accuracy of the placement. If that occurs, there is a danger that a satisfactory effect cannot be obtained when the number of LED chip is decreased.